Doctor blade for paper coating apparatus



Jan. 3, 1956 E. WARNER 2,729,192

DOCTOR BLADE FOR PAPER COATING APPARATUS Filed Dec. 51, 1952IIIIIIIIIIIIIIIIIIIIII 'IIIIIIIIIII/III entor 311v EDGAR WARNER GttornegUnited States Patent DOCTOR BLADE FOR PAPER COATING APPARATUS 8 Claims.(Cl. 118-417) This invention relates to the manufacture of coated paperfor high grade printing and other uses and consti- .tutes an improvementon the method set forth in U. S.

Patent No. 2,229,621 and it further relates to apparatus for practicingthe improved method. The invention has a field of especial utility inthe application of coating material comprising pigment and adhesive toone or both surfaces of a moving web of paper during the course of itsmanufacture on the paper making machine, but it is also useful forapplying the same or other types of coating to moving webs either on orofi? of the paper making machine.

The process of the aforesaid patent comprises the application of anexcess of fluid coating composition to a paper web, the formation of alayer of filter cake on the surface, and the wiping off of the liquidoverlying the filter cake. 'The patented process has been extensivelyand increasingly used ever since its introduction and has been found tobe highly satisfactory. Some inconveniences have, however, beenencountered in connection with its usein cases where the paper machineis functioning imperfectly due to causes completely aside from thecoating operation. For example, when there happens to beat loose edge orloose edges on the web or when someother portion of the width of the webruns slack for a greater or lesser time and is not corrected by alengthequalizing operation as set forth in the aforesaid patent, too great aweight of coating remains on the slack portions and either fouls thedrying cylinders or results in excessive coating weights on localizedportions of the web, or both. Also, when a break in the paper web occursbefore the web reaches the coater, it results in a loose end being drawnover the coating roll but not under tension as required to wipe off theexcess, which therefore remains on the loose end and fouls the dryingcylinders, which are often difiicult to clean. This can be extremelyaggravating at times when trouble with breaking of the web is beingencountered on the machine.

I have now found that I can eliminate the aforesaid diiliculties, eventhough the paper machine operation itself may be quite imperfect, andthat I can render the coating operation more uniform and satisfactory byremoving the surplus coating. composition and smoothing the coatingremaining on the web by means of a doctor or wiping blade which isflexible and provided with a spring support so that it is adapted toyield where the pressure of the web thereon is too strong and to springout and maintain properpressure on those parts of the web which wouldotherwise be slack. The Working edge of this doctor is desirably formedof a small diameter cylindrical rod adapted for rotation in its holder.In many cases I find it is of further advantage to provide a yieldingresilient carrier for the web, which supports the web where the springdoctor is pressed thereagainst. The support may be in the form of arubber covered roll or endless belt and may in many cases advantageouslybe moistened in order to cause the paper web to adhere 2 V thereto so,that it is placed .under additional tension Where it passes under thespring-pressed doctor, as described in U. S. Patent No. 2,534,321.

The invention will now be more fully described with reference to theaccompanying drawing in which:

Fig. 1 is a diagrammatic elevational view partly in section of a coatingdevice employing a flexible spring doctor in accordance with the presentinvention.

Fig. 2 is a similar view, also partly in section showing themodification in which the paper web is backed up ,by a yieldingresilient surfaced roll where the coated web is operated on by thespring docto Fig. 3 is a similar view of a modified form of the presentinvention.

Fig. 4 is a fragmentary sectional detail of the rotating doctor and itsholder, drawn to a much larger scale.

Fig. 5 is a fragmentary view illustrating means for' rotating the doctorrod by power applied at each end thereof.

Referring first to Fig. 1, the paper web 11 to be coated, passes over aroll 12 which is rotating in either direction at a suitable speed,dipping into liquid coating material 13 contained in a pan or trough 14.An excess of coating material is thereby carried up and deposited on theweb. A small diameter cylindrical doctor rod 20 is held in acorrespondingly formed holder 22 by spring pressure of a clip 23, aswill be more clearly seen in Fig. 4. Both holder and clip are continuousthroughout the working length of rod 20 and are arranged to expose anarcuate portion 25 of the cylindrical surface of rod 20, which serves asthe working edge of the doctor. Holder 22 and clip 23 togetherconstitute what is, in the appended claims, called a socket whichembraces the doctor rod around substantially more than half of itscircumference. The rod 26 and holders 22 and 23 are carried by springmount 24. This may be made of a single sheet of spring metal or aplurality of'strips mounted side by side, as desired. They should not,however, be widely separated since i have found it to be desirable thatthe springing action be substantially evenly distributed across the fullwidth of the paper web rather than being applied at the ends or atconsiderable intervals along the length of rod 29. The spring 24 is ofsuch stiffness and is so formed and mounted that changes in fiexure ofspring 24, which result from changes in web tension such as occur duringoperation, cause relatively small percentage changes in the pressure ofrod 20 against the paper web. It is desir-able that the formation andmounting of spring 24 be such that the directions ofmovements of rod 20,resultingfrom changes in fiexure of spring 24, form angles of not morethan about 45 deg, or at most 60 deg, with the direction of theresultant of the tensile forces on the web as it bends over rod'20.

If it is also desired to prevent the heavy coatings on broken ends of;the web, the arrangement illustrated in Fig. 2, may be used. Thisarrangement differs from that in Fig. l in that the p"per web is heldagainst the doctor rod 20 by a roll 3b which is provided with a yieldingresilient covering 31, of rubber or the like. In this case the path ofthe paper may remain as shown in Fig. l,

or it may advantageously be changed, as shown in Fig. 2,

' so that the paper wraps the roll 30 to a material extent before itpasses on to the drying operation. In either event, the paper willalways be held against doctor rod 20 regardless of whether or not thereis any externally applied tension on theweb, and the still liquid excessof coating will be wiped o of the web to the very end, even though theend is not held back, and fouling of the dryers is thereby prevented.The supporting spring 24 for doctor rod 20 may advantageously, in thiscase, be mounted at a different angle, but as before, the direction ofmovement of rod due to fiexure of spring 24 should make a large anglewith the path of the web. The spring metal portion 24 of the mounting,whether in the form of a single sheet of spring metal or a plurality ofstrips mounted side by side, as hereinbefore set forth in describingFig. l of the drawing, and whether bent as shown in Fig. l or straightas described in connection with the alternative embodiments of theinvention illustrated in Figs. 2 and 3, is hereinafter, in the appendedclaims, called a flexible plate-like structure of resilient material.

If the path of the paper web is made such that it wraps roll as shown inFig. 2, I find it advantageous to moisten the surface of the yieldingresilient cover 31 of roll 30 so that the paper web will adhere slightlyto the surface as it passes under doctor rod 20. This, as described inPatent No. 2,534,321 serves to provide a local increase in tension onthe web where it passes under doctor rod 20, due to the indentation ofthe surface of the rubber cover 31 on roll 30, and the consequentelongation of the surface in the immediate vicinity of the doctor. Thismoistening may be accomplished in any desired manner as, for example, bya shower 35 which serves to maintain a pool 36 of water between roll 30and another roll 37 held thereagainst by pressure which may be adjustedto give the desired degree of moistening.

Another alternative embodiment of the present invention is illustratedin Fig. 3. The doctor rod 20, and its spring mount 24 retain essentiallythe relationship to the yielding resilient covering 31 on roll 30, andthe paper web supported thereby as described in connection with Fig. 2.It differs in its position relative to the vertical so that in thismodification the paper is moving downwardly where it passes under thedoctor. This makes it possible to eliminate the applying roll 12 andapply the coating material directly to the paper by means of a showerpipe 40. The coating so supplied forms a pool in advance of the doctor,through which pool one surface of the paper passes before reaching thedoctor. In this figure I have also shown screws 42 which may, ifdesired, be provided at intervals along the length of the doctor mountso that the pressure of rod 20 on roll 30 may be locally adjusted ifdesired. The roll 37 may in this case be conveniently located beneaththe roll 30 and the water shower 45 directed into the nip between thesetwo rolls, as illustrated. Surplus water runs down into pan 46 and iscarried away by drain 47. Also shown in this embodiment is a doctor 48which removes surplus water, coating material which runs out beyond theedges of the paper web, and broken paper which might otherwise tend towrap around the roll. With the device as illustrated, the angle betweenthe doctor blade and the surface of roll 30 is advantageously of theorder of magnitude of 45 deg. since if it is much less than this it doesnot leave room for free circulation and flowback of the liquid coatingmaterial doctored ofl? of the web, and if it is much larger than this,the pressure exerted by the roll on the blade is directed too nearlydirectly towards the support so that the flexure of the spring tocompensate a given difference in level of the paper support surfacebecomes excessive.

The doctor rod 20 has been stated to be adapted to rotate in its holder.While it may be turned manually or mechanically through a part of arevolution at definite or indefinite intervals in order to distributethe wear and prolong its life, it is preferably rotated continuously atslow speed to distribute the wear more uniformly and to carry away lumpsof coating or foreign matter which might lodge on a stationary doctorand cause scratches in the surface of the coated web. The speed ofrotation is desirably slow and need be only enough to keep the rod incontinuous motion. A surface speed of about ft. per minute has beenfound satisfactory. This rotation is effective for the desired purposeregardless of direction, but it is generally preferred to rotate the rodcounter to the direction of movement of the paper thereover.

The diameter of the rod 20 should be determined with reference to anumber of factors. In the first place, the smaller the diameter of therod 20, the greater its flexibility and the greater its ability tofollow variations in web tension and conform to irregularities in thebacking roll surface, in response to the springing action provided inaccordance with the present invention. It should also be recognized thatdoctors with cylindrical working surfaces tend to draw the coatingmaterial into parallel ridges where it parts from the doctor surface.These ridges are finer and less noticeable the smaller the diameter ofthe cylindrical working surface of the doctor. These ridges areunimportant in the case of coatings which will flow out after beingapplied. In the case of coatings comprising mineral pigment andadhesive, however, such as are used for high grade printing papers, whenspread into the thickness used for that purpose, ridges and otherirregularities will not flow out to any appreciable extent. It isconsequently necessary, when using such coatings, to avoid the formationof ridges which are Wide and high enough to be objectionable. I havefound that doctor rods larger than about inch in diameter ordinarilyleave ridges too prominent to be acceptable in high grade printingpaper, though larger rods can be used when the smoothness re quirementsare less critical. For maximum smoothness of this type of coatings onthe surface of the paper, therefore, it is desirable to have thediameter of rod 20, as small as possible. On the other hand, rods muchless than about /3 inch in diameter are difficult to enclose and yetleave sufiicient working surface exposed. Further, small rods,especially when the working length is considerable, do not have adequatestiffness or strength to resist the torsional stresses involved in usesince, if the rod holders 22, 23 are sufiiciently close-fitting toprevent coating material from being carried around the rod 20 as itrotates, they impose a certain frictional resistance to rotation of therod. When the diameter of the rod is too small relative to its length,irregularities in the coated surface of the paper are found to be formeddue, it is thought, to excessive torsional vibrations in the rod. These,I find, can be reduced and smaller rods used for any given length (i. e.width of paper web) by applying the driving force to both ends of therod simultaneously. This may be accomplished by any suitable knownmethod as, for example, that illustrated in Fig. 5. In this case drivingforce is applied to each end of rod 20 by a flexible connection such asa flexible shaft 51 driven as by chain drives 52 from a relatively largediameter cross shaft 53, which synchronizes the drives on opposite endsof rod 20, and is driven as by a suitable motor reducer 54. The chaindrives 52 and the fixed ends of flexible shafts 51 are shown assupported in suitable bearings 55. If desired, the cross shaft can beeliminated and the two ends may be separately driven by separate motorreducers. In practice I have found a rod 20 of A inch diameter tooperate highly satisfactorily in lengths up to about 15 feet, whendriven from both ends. In devices for coating webs of lesser width, rodsof diameters down to about /8 inch, or even less, can be usedsatisfactorily and give excellent results.

These doctor rods should not corrode readily in contact with thecoatings used and they should be hard enough to wear for a satisfactorylength of time before requiring replacement. For use with the aqueous,mineral pigment coatings such as those used in the manufacture ofprinting paper, I find tool steel rods plated with about 0.002 inch ofchromium to operate without requiring replacement for wear or corrosion,for periods of continuous operation of 300 to 400 hours.

The spring mount 24 should be stiff enough to provide pressures of up toabout 5 to 10 pounds per linear inch of rod 20 with reasonabledeflection, but should be flexible enough to adjust itself to compensatefor the maximum expected variation in web length, or surface level ofthe backing roll, without significant change in the pressure which itexerts on the web.

For the rubber cover 31 on roll 30, a Pusey and Jones density of from 80to 120 has been found satisfactory with doctor rods of about inchdiameter.

As one example of the practice of the present invention, the cover 31 onbacking roll 30 was about inch thick and had a Pusey and Jones densityof about 85. Doctor rod 20 was of polished tool steel inch in diameterplated with hard chromium to a thickness of about 0.002 inch on theradius. The spring mount 24 was of 0.035 inch thick spring steel andprojected approximately 2 /2 inches from its support. It was pressedagainst the surface of the backing roll with a pressure of 6 to 7 poundsper linear inch and was rotated continuously in a direction counter tothe direction of movement of the paper web, at a speed of about 10revolutions per minute, by means of a geared motor reducer on each end.The roll 30 was about 174 inches in length while the length of thedoctor rod holders 22 and 23 and spring mount 24 were slightly longer,and the working length of rod 20 was slightly less. The rod, however,projected beyond the ends of the holders 22, 23, and was connected byflexible connectors to the driving means. In this case the parts werearranged and the paper threaded through the device as illustrated inFig. 3. The coating was an aqueous composition composed primarily ofcalcium carbonate, coating clay, and casein adhesive dispersed in waterand containing about 47 per cent total solids. The paper web was about156 inches in width and was composed primarily of sized bleached sulfatepulp, and passed directly from the first dryer section of the papermaking machine, at a speed of about eight hundred feet per minute, overroll 30 of the coating device which applied the coating to the wire sideof the web. From the coater the web passed over another section ofdryers and through another coating device of the same kind which applieda second coat to the same side of the web. The web then passed overfurther dryers until the drying was completed. The coated paper was thencalendered. The resulting product had a coated weight of about 60 poundsper ream 500 sheets 25 x 38 inches) while the coating weight was aboutpounds per ream. It was a high quality of coated-one-side paper with ahighly uniform coated surface, free from visible ridges and blemishes,and suitable for receiving high grade printing.

The flexible blade with the working edge formed by a rotating rod hasbeen found to compensate for the normally encountered variations inlength of a paper web when operating on an unbacked web as illustratedin Fig. 1. When operating on a web backed by a rubber covered roll asillustrated in Figs. 2 and 3, it has been found to compensate for dentsin the roll surface such as are sometimes caused when crumpled scraps ofa broken paper web pass between rolls under pressure, or flats formedwhen a rubber covered roll is carelessly allowed to stand resting on itssurface rather than fully supported on its journal-bearings. When thepaper passing over such a blade is backed up by a roll as illustrated inFigs. 2 and 3, the present improvement not only compensates forirregularities in length of a paper web, but also completely preventscarrying through of excessive amounts of wet coating on loose ends of apaper web, and completely prevents the former troublesome fouling of thedryers.

If it is desired to apply the exact amount of coating to the paper webby a printing operation, instead of applying an excess to the web anddoctoring off the surplus as described, the apparatus illustrated inFig. 3 can be used by eliminating the water shower 40 and passing thepaper web from left to right between rolls 30 and 37, instead of passingit around roll 30 and under doctor rod as shown. The doctor then spreadsa uniform film over the surface of the rubber cover 31 on roll 30 andthis film of coating is then transferred to the surface of the paper webby the pressure in the nip between rolls 30 and37. Because of theimpervious surface of the rubber cover 31, no filter cake is formed onits surface and the film of coating left on it by the doctor isconsequently thinner than that left by the same doctor on the surface ofthe paper web. I havefound its possible, however, by using a wire woundor threaded doctor rod, to leave as heavy a coating as desired on theroll 30 for transfer to the paper web. The number of threads required.is determined by the thickness of coating layer desired. Coatings of athickness commonly used on printing paper can be applied by using rodshaving 30 to threads, of approximately Whitworth form, per inch. I havefound it possible by this means to successfully coat paper by theprinting method with a simpler type of apparatus than that which hasheretofore been considered necessary for this purpose.

I claim:

1. A device for making coated printing paper wherein doctoring means isprovided for smoothing and limiting the thickness of the coating, whichdoctoring means comprises: a flexible cylindrical doctor rod; a flexiblesocket in which said rod is rotatably mounted, which socket embracessaid rod around more than half of its circumference and is continuousthroughout the working length of the rod; a spring mounting of uniformstiffness throughout the entire working length of said doctor rod, saidmounting comprising a flexible plate-like structure of resilientmaterial, to one edge of which plate-like structure said flexible socketis fixedly attached throughout its length; and a rigid support on whichthe other edge of said plate-like structure is fixedly mountedthroughout its length, whereby the position of the doctor rod and thecorresponding degree of flexure of said plate-like structure, at anytime, depend upon the degree of pressure against said doctor rod at thattime.

2. A device as defined in claim 1 wherein the path of the paper web ischanged materially in direction at the doctor rod, by contact therewith,whereby the position of the doctor rod is determined by the equilibriumof the forces of tension in the paper web and the forces of resiliencein the rod and its flexible socket and platelike mounting structure.

3. A device as defined in claim 1 wherein a roll with a yieldingresilient covering is mounted adjacent the doctor rod in a position tosupport the paper web against pressure exerted thereon by said doctorrod.

4. A device as definied in claim 1 wherein the platelike structure issecured to its support in such proximity and angular relationship to thesurface on which coating is being doctored that it is adapted toconstrain movements of the doctor rod, from its normal operatingposition, to take place at an angle of not more than about 45 degreeswith the normal to the surface on which coating is being doctored at themidpoint of the contact of said surface with the doctor rod.

5. A device as defined in claim 1 wherein the platelike structure isfixed to its support in such proximity and angular relationship to thesurface on which the coating is being doctored, and possesses suchdegree of flexibility that, when the doctor is in operating positionagainst the surface on which the coating is being doctored, theplatelike structure is flexed to a degree which provides doctoringpressure between said rod and said surface and, during normal operation,maintains the doctor rod in operative position against said surface withonly small percentage changes in the degree of fiexure and in theresulting pressure between said rod and said surface.

6. A device as defined in claim 3 wherein the doctor rod is betweenone-eighth and three-eighths inch in diameter and is pressed against theroll with a pressure of between five and ten pounds per linear inch.

7. A device as defined in claim 1 wherein means is provided forcontinuously rotating the doctor rod in a direction opposite to thedirection of movement, beneath References Cited in the file of thispatent UNITED STATES PATENTS Potdevin Aug. 7, 1934 Grant Nov. 15, 1938Kauppi et a1. Nov. 9, 1943 Trist Jan. 30, 1945 Taylor Dec. 19, 1950Warner June 3, 1952

